The present invention relates to a method for superplastic forming by internal pressure.
"Superplasticity" refers to the phenomenon that some materials, when subjecting to a specific strain rate at a specific temperature, exhibit very high tensile elongations and controlled thinning without rupture or necking. Materials having the "superplasticity" property include titanium-based alloy, aluminum-based alloy, copper-based alloy, iron-based alloy and nickel-based alloy. For example, Ti6Al4V alloy and Ti6Al6V2Sn alloy are superplastic materials which have been extensively used in aircraft materials. 7475Al--Zn--Mg alloy, 2090 Al--Li alloy and 8090 Al--Li alloy are superplastic materials which are now attracting the attention of the aeronautical industry. A large amount of superplastic materials have been rapidly developed. In the "International Symposium on superplasticity of top materials "held in Osaka, Japan in June. 1991, many materials including metal, non-metal, metal composite, structural ceramics, high temperature superconductor ceramics and ceramic composite were reported to possess superplasticity.
The superplastic forming techniques capitalizing on superplasticity of selected materials have many advantages including: very complex shapes and deep drawn parts can be readily formed, less energy is required to work the superplastic forming due to the low deformation stress for forming, thereby minimizing tool deformation and wear. With the above advantages, superplastic forming is considered to be most suitable for processing titanium-based alloys which are conventionally hard to process by known forming techniques.
C. H. Hamilton et al. (U.S. Pat. Nos. 4,181,000; 4,354,369), D. S. Fields et al.(U.S. Pat. No. 3,340,101) and B. B. Hundy (U.S. Pat. No. 3,595,060) disclose methods for superplastic forming metal blanks in which inert gases, for example argon gas, are used to apply fluid pressure loading across the metal blanks when superplastic forming is carried out. The pressure of the argon is about 100-300 psi. D. B. Laycock et al. in their U.S. Pat. No. 4,045,986 disclose a superplastic forming method for shaping superplastic alloy sheet into a finished body having an approximately uniform wall thickness. The method includes applying argon gas to urge a peripherally clamped sheet against a female molding surface to form a partially shaped bubblelike preform and then advancing a male mold toward the sheet from the other side and applying reverse pressure to cause the preform to conform to the shape of that mold. U.S. Pat. No. 3,920,175 issued to C. H. Hamilton discloses a method for making a metal structure by superplastic forming metals with concurrent diffusion bonding. In this method, an inert gas is also used.
As discussed above, conventional methods of superplastic forming involve the use of expensive and high purity argon gas to apply pressure to one side of the metal blank to be superplastic formed. This requires pipelines, flow control values and apparatus for generating pressure, causing an increase in the difficulty of designing the tooling and workpiece structure.
In addition, a German Company, Messerschmitt-Bolkow-Blohm (MBB), has exploited a no-die superplastic forming method for forming hollow spherical bodies. The method includes cutting a Ti6Al4V alloy plate into two disk blanks in which one disk blank is provided with a hole for connecting to a pipe member; bonding the rims of the two disk blanks by welding; and heating the two disk blanks at a temperature of 925.degree. C. while argon gas is instilled into through the pipe member. Hollow spherical bodies are therefore formed without using a die. The above no-die superplastic forming technique has been commercialized.
The above no-die superplastic forming still uses high-cost argon gas and requires connecting welding pipe members to one disk blank for flowing in the argon gas. The finished spherical bodies are therefore not perfectly spherical, sometimes resulting in unacceptable distortion if they are to be used in high precision structural components. Furthermore, welding pipe members for argon gas and cutting off the pipe member after superplastic forming will inevitably increase the manufacturing cost.